Neurotransmitter Systems: Catecholamines, Glutamate, GABA, and Acetylcholine

Vocabulary practice cards providing definitions, chemical pathways, receptor mechanisms, and pharmacological interventions for catecholamines (dopamine, norepinephrine), amino acid transmitters (glutamate, GABA), and acetylcholine.

Catecholamines

A major subcategory of monoamine and biogenic amine neurotransmitters synthesized in presynaptic terminals from the amino acid tyrosine, characterized by a core structure containing a catechol ring (a benzene ring with two hydroxyl groups) and a nitrogen-containing amine side group.

Tyrosine Hydroxylase (TH)

The enzyme responsible for the first step of catecholamine biosynthesis, converting the dietary amino acid tyrosine into DOPA by adding a hydroxyl group to the aromatic benzene ring.

Aromatic Amino Acid Decarboxylase (AADC)

The cytosolic enzyme that converts DOPA into dopamine by catalyzing the removal of its carboxyl group ($-\text{COOH}$).

Dopamine $\beta$-Hydroxylase (DBH)

The enzyme responsible for synthesizing norepinephrine from dopamine by adding a hydroxyl group to the dopamine molecule.

Vesicular Monoamine Transporter (VMAT)

A transporter protein embedded in synaptic vesicle membranes that packages dopamine and norepinephrine into small synaptic vesicles for quantal release, protecting them from enzymatic degradation within the terminal.

Reserpine

A drug that pharmacologically inhibits the vesicular monoamine transporter (VMAT), preventing the vesicular packaging and storage of both dopamine and norepinephrine.

Catecholamine Autoreceptors

Presynaptic membrane receptors located on terminals and cell bodies that regulate neurotransmission by decreasing presynaptic $\text{Ca}^{2+}$ influx, thereby inhibiting calcium-dependent vesicular exocytosis of dopamine or norepinephrine.

Catechol-O-Methyltransferase (COMT)

A primary metabolic enzyme in the synaptic cleft that inactivates catecholamines by transferring a methyl group to one of the hydroxyl groups of the catechol ring.

Monoamine Oxidase (MAO)

A metabolic enzyme responsible for catecholamine degradation that inactivates the neurotransmitter molecule by replacing its amine group.

Dopamine Transporter (DAT) and Norepinephrine Transporter (NERT)

Terminal membrane transport proteins responsible for the high-affinity reuptake of dopamine and norepinephrine from the synaptic cleft back into the presynaptic cytoplasm for repackaging or metabolic degradation.

A9 Dopamine Cell Group

Dopaminergic cell bodies situated in the substantia nigra that project through the nigrostriatal pathway to the striatum (specifically the putamen in humans), playing a central role in fine motor control and representing the primary cell population degenerate in Parkinson's disease.

A10 Dopamine Cell Group

Dopaminergic neurons located in the ventral tegmental area (VTA) that project to the basal forebrain, cortex, and hippocampus via the mesolimbic and mesocortical pathways, mediating reward, cognition, and sensory recognition, and implicated in psychiatric disorders such as schizophrenia.

A6 Noradrenergic Cell Group

A dense group of approximately $3000$ norepinephrine-producing neurons situated in the locus coeruleus (LC) behind the brainstem that project diffusely across the forebrain, cerebellum, and spinal cord to mediate vigilance and attention.

$\text{D}_1$-Like Dopamine Receptors

Metabotropic dopamine receptors ($\text{D}_1$ and $\text{D}_5$) coupled to $\text{G}_s$ proteins whose activation stimulates adenylyl cyclase, elevates intracellular cAMP, and activates protein kinase A (PKA) to close $\text{K}^+$ channels and open $\text{Ca}^{2+}$ channels, resulting in neuronal depolarization.

$\text{D}_2$-Like Dopamine Receptors

Metabotropic dopamine receptors ($\text{D}_2$, $\text{D}_3$, and $\text{D}_4$) coupled to $\text{G}_i$ proteins that inhibit adenylyl cyclase, suppress cAMP generation, and fail to activate PKA, maintaining $\text{K}^+$ channels open and $\text{Ca}^{2+}$ channels closed to produce hyperpolarization.

Apomorphine

A non-selective dopamine receptor agonist that stimulates both $\text{D}_1$ and $\text{D}_2$ families of receptors, producing behavioral stimulation, increased locomotion, and grooming behaviors comparable to cocaine.

Catalepsy

A behavioral state defined by the absence of voluntary or spontaneous movement and muscular rigidity, typically elicited experimentally by administering dopamine $\text{D}_1$ or $\text{D}_2$ receptor antagonists.

$\alpha_1$-Adrenoceptor

A postsynaptic metabotropic norepinephrine receptor that couples to the $\text{IP}_3$ and diacylglycerol (DAG) second messenger cascade, elevating intracellular $\text{Ca}^{2+}$ to stimulate cellular excitation and arousal.

$\alpha_2$-Adrenoceptor

A metabotropic norepinephrine receptor that inhibits adenylyl cyclase and lowers cAMP; functions postsynaptically to cause hyperpolarization by opening $\text{K}^+$ channels, and presynaptically as an autoreceptor on terminals and somata to suppress further norepinephrine release.

$\beta$-Adrenoceptors

Postsynaptic metabotropic adrenoceptors ($\beta_1$ and $\beta_2$) coupled to $\text{G}_s$ proteins that activate adenylyl cyclase and increase intracellular cAMP levels to stimulate target cell excitability, cardiac contraction, and bronchial relaxation.

Glutamate (GLU)

The principal fast-acting excitatory amino acid neurotransmitter in the central nervous system, synthesized predominantly from glutamine or dietary glucose metabolites and utilized by cortical pyramidal output neurons, hippocampal circuits, and the cerebellum.

Glutaminase

A presynaptic mitochondrial enzyme in glutamatergic neurons that synthesizes neurotransmitter glutamate by cleaving an amine group from the precursor glutamine.

Vesicular Glutamate Transporters (VGLUT1–3)

Transporter proteins exclusively found on the synaptic vesicles of glutamatergic neurons that concentrate glutamate into vesicles for calcium-dependent quantal release, with individual neurons generally expressing either VGLUT1 or VGLUT2.

Excitatory Amino Acid Transporters (EAAT1–3)

High-affinity reuptake transporters responsible for rapid clearance of glutamate from the synaptic cleft, where EAAT1 and EAAT2 reside on surrounding astrocytes and EAAT3 is situated directly on the presynaptic neuronal membrane.

Glutamine Synthetase

An astrocytic enzyme that converts taken-up glutamate back into non-neuroactive glutamine by attaching an amine group, completing an essential step of the glutamate-glutamine recycling partnership between glia and neurons.

AMPA Receptor

A widely expressed ionotropic glutamate receptor assembled from GluR1–4 subunits that functions as a ligand-gated $\text{Na}^+$ channel, mediating the majority of fast excitatory synaptic transmission and baseline EPSP generation.

Kainate Receptor

An ionotropic glutamate receptor composed of KA1–2 and GluR5–7 subunits that acts as a selective ligand-gated $\text{Na}^+$ channel to generate fast excitatory depolarizations.

NMDA Receptor

A specialized ionotropic glutamate receptor permeable to $\text{Na}^+$ and $\text{Ca}^{2+}$ that requires the simultaneous binding of glutamate and a co-agonist (glycine or D-serine), as well as membrane depolarization to displace a tonic channel-blocking $\text{Mg}^{2+}$ ion.

Phencyclidine (PCP) Binding Site

A non-competitive antagonist binding pocket located deep within the NMDA receptor ion pore that blocks ion permeation only when the receptor is open and the antagonist is bound.

Long-Term Potentiation (LTP)

A persistent enhancement in synaptic efficacy induced by high-frequency presynaptic stimulation (tetanus), initiated by NMDA receptor-mediated $\text{Ca}^{2+}$ entry, which activates CaMKII to phosphorylate AMPA receptors and drive the insertion of new AMPA receptors into the postsynaptic density.

Excitotoxicity

Neuronal damage and death resulting from excessive exposure to glutamate and pathological overactivation of NMDA receptors, precipitating either rapid lysis or delayed osmotic swelling triggered by massive $\text{Ca}^{2+}$ and $\text{Na}^+$ influx.

$\gamma$-Aminobutyric Acid (GABA)

The primary fast-acting inhibitory amino acid neurotransmitter in the central nervous system, utilized by approximately half of all neurons in the cerebral cortex, striatum, hippocampus, and cerebellum.

Glutamic Acid Decarboxylase (GAD)

The rate-limiting biosynthetic enzyme in GABAergic neurons that synthesizes GABA by cleaving a carboxyl group from glutamate.

Vesicular GABA Transporter (VGAT)

The specific transporter protein embedded in synaptic vesicles of inhibitory neurons that packages synthesized GABA into vesicles for regulated quantal exocytosis.

GABA Transporters (GAT1–3)

Membrane proteins mediating synaptic GABA reuptake, wherein GAT1 and GAT2 are distributed on both neurons and astrocytes, while GAT3 is located strictly on astrocytes.

Tiagabine (Gabitril)

An anticonvulsant medication that selectively blocks the GAT1 transporter, elevating synaptic GABA availability to enhance central inhibitory neurotransmission.

GABA Transaminase (GABA-T)

An enzyme present in neurons and astrocytes that degrades GABA by transamination with $\alpha$-ketoglutarate, yielding glutamate and succinic semialdehyde (succinic acid pathway).

$\text{GABA}_A$ Receptor

A pentameric ionotropic receptor (predominantly composed of 2\text{\ensuremath{\alpha}}, 2\text{\ensuremath{\beta}}, 1\text{\ensuremath{\gamma}}) acting as a ligand-gated $\text{Cl}^-$ channel that mediates fast inhibitory hyperpolarization and accounts for over $85\%$ of CNS GABA receptors.

$\text{GABA}_B$ Receptor

An obligate heterodimeric ($\text{B}_1, \text{B}_2$) metabotropic receptor coupled to $\text{G}_i$ proteins that suppresses adenylyl cyclase and cAMP production, keeping $\text{K}^+$ channels open and $\text{Ca}^{2+}$ channels closed to produce prolonged inhibitory hyperpolarization.

Choline Acetyltransferase (ChAT)

The rate-limiting biosynthetic enzyme located in presynaptic cholinergic terminals that transfers an acetyl group from acetyl-CoA to dietary choline to synthesize acetylcholine.

Vesicular Acetylcholine Transporter (VAChT)

A vesicle membrane protein that loads acetylcholine into synaptic vesicles for quantal release, serving as the pharmacological target blocked by the drug vesamicol.

Vesamicol

An experimental agent that selectively inhibits the vesicular acetylcholine transporter (VAChT), preventing vesicular storage and subsequent exocytosis of acetylcholine.

Hemicholinium-3 (HC-3)

A pharmacological compound that selectively blocks presynaptic high-affinity choline transporters, preventing choline uptake and reducing ongoing acetylcholine biosynthesis.

Acetylcholinesterase (AChE)

A membrane-bound hydrolytic enzyme localized to presynaptic and postsynaptic membranes that rapidly terminates cholinergic signaling by breaking down acetylcholine into choline and acetic acid.

Physostigmine

A reversible acetylcholinesterase inhibitor that crosses the blood-brain barrier, resulting in acetylcholine accumulation in both the central and peripheral nervous systems.

Neostigmine

A synthetic reversible acetylcholinesterase inhibitor that does not cross the blood-brain barrier, commonly used to treat peripheral neuromuscular conditions with low cholinergic tone such as myasthenia gravis.

Sarin

A synthetic organophosphate nerve agent that irreversibly inhibits acetylcholinesterase, producing catastrophic synaptic accumulation of acetylcholine and death by asphyxiation due to diaphragm paralysis.

Basal Forebrain Cholinergic System (BFCS)

A widespread ascending cholinergic network originating from the medial septum, diagonal band nuclei, substantia innominata, and nucleus basalis that innervates the neocortex and hippocampus to modulate attention, learning, and cognition.

Nicotinic Acetylcholine Receptor (nAChR)

A pentameric ionotropic receptor (2\text{\ensuremath{\alpha}}, 1\text{\ensuremath{\beta}}, 1\text{\ensuremath{\gamma}}, 1\text{\ensuremath{\delta}} ) permeable to $\text{Na}^+$ and $\text{Ca}^{2+}$, requiring the simultaneous binding of two acetylcholine molecules to its $\alpha$ subunits to mediate fast excitatory depolarization.

Depolarization Blockade

A state of physiological electrical inexcitability occurring when continuous agonist stimulation maintains a cell in an uninterrupted depolarized state, preventing repolarization to its resting potential ($-70\,\text{mV}$) and causing voltage-gated ion channels to remain inactivated.

Succinylcholine

A durable nicotinic acetylcholine receptor agonist resistant to acetylcholinesterase cleavage that induces continuous muscle depolarization and depolarization blockade, clinically utilized as a muscle relaxant in surgical anesthesia.

Curare

A competitive nicotinic acetylcholine receptor antagonist that prevents acetylcholine binding at the neuromuscular junction, inducing flaccid muscle paralysis and respiratory failure.

Muscarinic Acetylcholine Receptors (mAChR)

A family of five metabotropic receptor subtypes ($\text{M}_1\text{–}\text{M}_5$) acting via $\text{G}_q$ (activating $\text{IP}_3/\text{DAG}$) or $\text{G}_i$ (inhibiting cAMP), regulating cognitive processing in the brain as well as parasympathetic stimulation of salivation, lacrimation, and sweating in the periphery.

Catecholamines

A major subcategory of monoamine and biogenic amine neurotransmitters synthesized in presynaptic terminals from the amino acid tyrosine, characterized by a core structure containing a catechol ring (a benzene ring with two hydroxyl groups) and a nitrogen-containing amine side group.

Tyrosine Hydroxylase (TH)

The enzyme responsible for the first step of catecholamine biosynthesis, converting the dietary amino acid tyrosine into DOPA by adding a hydroxyl group to the aromatic benzene ring.

Aromatic Amino Acid Decarboxylase (AADC)

The cytosolic enzyme that converts DOPA into dopamine by catalyzing the removal of its carboxyl group ($-\text{COOH}$).

Dopamine $\beta$-Hydroxylase (DBH)

The enzyme responsible for synthesizing norepinephrine from dopamine by adding a hydroxyl group to the dopamine molecule.

Vesicular Monoamine Transporter (VMAT)

A transporter protein embedded in synaptic vesicle membranes that packages dopamine and norepinephrine into small synaptic vesicles for quantal release, protecting them from enzymatic degradation within the terminal.

Reserpine

A drug that pharmacologically inhibits the vesicular monoamine transporter (VMAT), preventing the vesicular packaging and storage of both dopamine and norepinephrine.

Catecholamine Autoreceptors

Presynaptic membrane receptors located on terminals and cell bodies that regulate neurotransmission by decreasing presynaptic $\text{Ca}^{2+}$ influx, thereby inhibiting calcium-dependent vesicular exocytosis of dopamine or norepinephrine.

Catechol-O-Methyltransferase (COMT)

A primary metabolic enzyme in the synaptic cleft that inactivates catecholamines by transferring a methyl group to one of the hydroxyl groups of the catechol ring.

Monoamine Oxidase (MAO)

A metabolic enzyme responsible for catecholamine degradation that inactivates the neurotransmitter molecule by replacing its amine group.

Dopamine Transporter (DAT) and Norepinephrine Transporter (NERT)

Terminal membrane transport proteins responsible for the high-affinity reuptake of dopamine and norepinephrine from the synaptic cleft back into the presynaptic cytoplasm for repackaging or metabolic degradation.

A9 Dopamine Cell Group

Dopaminergic cell bodies situated in the substantia nigra that project through the nigrostriatal pathway to the striatum (specifically the putamen in humans), playing a central role in fine motor control and representing the primary cell population degenerate in Parkinson's disease.

A10 Dopamine Cell Group

Dopaminergic neurons located in the ventral tegmental area (VTA) that project to the basal forebrain, cortex, and hippocampus via the mesolimbic and mesocortical pathways, mediating reward, cognition, and sensory recognition, and implicated in psychiatric disorders such as schizophrenia.

A6 Noradrenergic Cell Group

A dense group of approximately $3000$ norepinephrine-producing neurons situated in the locus coeruleus (LC) behind the brainstem that project diffusely across the forebrain, cerebellum, and spinal cord to mediate vigilance and attention.

$\text{D}_1$-Like Dopamine Receptors

Metabotropic dopamine receptors ($\text{D}_1$ and $\text{D}_5$) coupled to $\text{G}_s$ proteins whose activation stimulates adenylyl cyclase, elevates intracellular cAMP, and activates protein kinase A (PKA) to close $\text{K}^+$ channels and open $\text{Ca}^{2+}$ channels, resulting in neuronal depolarization.

$\text{D}_2$-Like Dopamine Receptors

Metabotropic dopamine receptors ($\text{D}_2$, $\text{D}_3$, and $\text{D}_4$) coupled to $\text{G}_i$ proteins that inhibit adenylyl cyclase, suppress cAMP generation, and fail to activate PKA, maintaining $\text{K}^+$ channels open and $\text{Ca}^{2+}$ channels closed to produce hyperpolarization.

Apomorphine

A non-selective dopamine receptor agonist that stimulates both $\text{D}_1$ and $\text{D}_2$ families of receptors, producing behavioral stimulation, increased locomotion, and grooming behaviors comparable to cocaine.

Catalepsy

A behavioral state defined by the absence of voluntary or spontaneous movement and muscular rigidity, typically elicited experimentally by administering dopamine $\text{D}_1$ or $\text{D}_2$ receptor antagonists.

$\alpha_1$-Adrenoceptor

A postsynaptic metabotropic norepinephrine receptor that couples to the $\text{IP}_3$ and diacylglycerol (DAG) second messenger cascade, elevating intracellular $\text{Ca}^{2+}$ to stimulate cellular excitation and arousal.

$\alpha_2$-Adrenoceptor

A metabotropic norepinephrine receptor that inhibits adenylyl cyclase and lowers cAMP; functions postsynaptically to cause hyperpolarization by opening $\text{K}^+$ channels, and presynaptically as an autoreceptor on terminals and somata to suppress further norepinephrine release.

$\beta$-Adrenoceptors

Postsynaptic metabotropic adrenoceptors ($\beta_1$ and $\beta_2$) coupled to $\text{G}_s$ proteins that activate adenylyl cyclase and increase intracellular cAMP levels to stimulate target cell excitability, cardiac contraction, and bronchial relaxation.

Glutamate (GLU)

The principal fast-acting excitatory amino acid neurotransmitter in the central nervous system, synthesized predominantly from glutamine or dietary glucose metabolites and utilized by cortical pyramidal output neurons, hippocampal circuits, and the cerebellum.

Glutaminase

A presynaptic mitochondrial enzyme in glutamatergic neurons that synthesizes neurotransmitter glutamate by cleaving an amine group from the precursor glutamine.

Vesicular Glutamate Transporters (VGLUT1–3)

Transporter proteins exclusively found on the synaptic vesicles of glutamatergic neurons that concentrate glutamate into vesicles for calcium-dependent quantal release, with individual neurons generally expressing either VGLUT1 or VGLUT2.

Excitatory Amino Acid Transporters (EAAT1–3)

High-affinity reuptake transporters responsible for rapid clearance of glutamate from the synaptic cleft, where EAAT1 and EAAT2 reside on surrounding astrocytes and EAAT3 is situated directly on the presynaptic neuronal membrane.

Glutamine Synthetase

An astrocytic enzyme that converts taken-up glutamate back into non-neuroactive glutamine by attaching an amine group, completing an essential step of the glutamate-glutamine recycling partnership between glia and neurons.

AMPA Receptor

A widely expressed ionotropic glutamate receptor assembled from GluR1–4 subunits that functions as a ligand-gated $\text{Na}^+$ channel, mediating the majority of fast excitatory synaptic transmission and baseline EPSP generation.

Kainate Receptor

An ionotropic glutamate receptor composed of KA1–2 and GluR5–7 subunits that acts as a selective ligand-gated $\text{Na}^+$ channel to generate fast excitatory depolarizations.

NMDA Receptor

A specialized ionotropic glutamate receptor permeable to $\text{Na}^+$ and $\text{Ca}^{2+}$ that requires the simultaneous binding of glutamate and a co-agonist (glycine or D-serine), as well as membrane depolarization to displace a tonic channel-blocking $\text{Mg}^{2+}$ ion.

Phencyclidine (PCP) Binding Site

A non-competitive antagonist binding pocket located deep within the NMDA receptor ion pore that blocks ion permeation only when the receptor is open and the antagonist is bound.

Long-Term Potentiation (LTP)

A persistent enhancement in synaptic efficacy induced by high-frequency presynaptic stimulation (tetanus), initiated by NMDA receptor-mediated $\text{Ca}^{2+}$ entry, which activates CaMKII to phosphorylate AMPA receptors and drive the insertion of new AMPA receptors into the postsynaptic density.

Excitotoxicity

Neuronal damage and death resulting from excessive exposure to glutamate and pathological overactivation of NMDA receptors, precipitating either rapid lysis or delayed osmotic swelling triggered by massive $\text{Ca}^{2+}$ and $\text{Na}^+$ influx.

$\gamma$-Aminobutyric Acid (GABA)

The primary fast-acting inhibitory amino acid neurotransmitter in the central nervous system, utilized by approximately half of all neurons in the cerebral cortex, striatum, hippocampus, and cerebellum.

Glutamic Acid Decarboxylase (GAD)

The rate-limiting biosynthetic enzyme in GABAergic neurons that synthesizes GABA by cleaving a carboxyl group from glutamate.

Vesicular GABA Transporter (VGAT)

The specific transporter protein embedded in synaptic vesicles of inhibitory neurons that packages synthesized GABA into vesicles for regulated quantal exocytosis.

GABA Transporters (GAT1–3)

Membrane proteins mediating synaptic GABA reuptake, wherein GAT1 and GAT2 are distributed on both neurons and astrocytes, while GAT3 is located strictly on astrocytes.

Tiagabine (Gabitril)

An anticonvulsant medication that selectively blocks the GAT1 transporter, elevating synaptic GABA availability to enhance central inhibitory neurotransmission.

GABA Transaminase (GABA-T)

An enzyme present in neurons and astrocytes that degrades GABA by transamination with $\alpha$-ketoglutarate, yielding glutamate and succinic semialdehyde (succinic acid pathway).

$\text{GABA}_A$ Receptor

A pentameric ionotropic receptor (predominantly composed of $2\alpha, 2\beta, 1\gamma$) acting as a ligand-gated $\text{Cl}^-$ channel that mediates fast inhibitory hyperpolarization and accounts for over $85\%$ of CNS GABA receptors.

$\text{GABA}_B$ Receptor

An obligate heterodimeric ($\text{B}_1, \text{B}_2$) metabotropic receptor coupled to $\text{G}_i$ proteins that suppresses adenylyl cyclase and cAMP production, keeping $\text{K}^+$ channels open and $\text{Ca}^{2+}$ channels closed to produce prolonged inhibitory hyperpolarization.

Choline Acetyltransferase (ChAT)

The rate-limiting biosynthetic enzyme located in presynaptic cholinergic terminals that transfers an acetyl group from acetyl-CoA to dietary choline to synthesize acetylcholine.

Vesicular Acetylcholine Transporter (VAChT)

A vesicle membrane protein that loads acetylcholine into synaptic vesicles for quantal release, serving as the pharmacological target blocked by the drug vesamicol.

Vesamicol

An experimental agent that selectively inhibits the vesicular acetylcholine transporter (VAChT), preventing vesicular storage and subsequent exocytosis of acetylcholine.

Hemicholinium-3 (HC-3)

A pharmacological compound that selectively blocks presynaptic high-affinity choline transporters, preventing choline uptake and reducing ongoing acetylcholine biosynthesis.

Acetylcholinesterase (AChE)

A membrane-bound hydrolytic enzyme localized to presynaptic and postsynaptic membranes that rapidly terminates cholinergic signaling by breaking down acetylcholine into choline and acetic acid.

Physostigmine

A reversible acetylcholinesterase inhibitor that crosses the blood-brain barrier, resulting in acetylcholine accumulation in both the central and peripheral nervous systems.

Neostigmine

A synthetic reversible acetylcholinesterase inhibitor that does not cross the blood-brain barrier, commonly used to treat peripheral neuromuscular conditions with low cholinergic tone such as myasthenia gravis.

Sarin

A synthetic organophosphate nerve agent that irreversibly inhibits acetylcholinesterase, producing catastrophic synaptic accumulation of acetylcholine and death by asphyxiation due to diaphragm paralysis.